The present invention relates to a granular bed filtering arrangement, and more particularly to a granular bed system for filtering and collecting particulate matter of very small size (referred to hereinafter as "dust") from gaseous streams. The invention also pertains to a process of separating entrained dust from a gaseous stream, and for periodically cleaning the granular bed filter media in an automatic apparatus for performing a process which separates the dust from the filter media and collects the dust in a suitable container for disposal or recovery of materials. The granular bed filter media is also variously referred to as filter medium and granular material.
Granular bed filters have been used extensively to collect the dust from hot process exhaust systems. One type of granular bed filter used for this purpose is exemplified by those illustrated in Berz, U.S. Pat. Nos. 3,868,237 and 3,871,846. In such a filter, the dust-contaminated exhaust gas, after flowing through a cyclone separator, flows downwardly through a horizontally arranged granular filter bed in which the dust is entrapped. When the bed becomes dustladen, a backflush gas is blown upwardly through the bed to dislodge the collected dust, preferably in agglomerated chunks, and carry it into the cyclone for separation from the backflush gas. The backflush gas from the cyclone outlet rejoins the contaminated gas stream and thence to another granular bed filter of the system for cleaning.
Granular bed filters of this type which employ a stationary bed and a backflushing step operate very satisfactorily for many applications, for example, in cleaning exhaust gas streams from cement manufacturing plants. However, this type of system also possesses certain inherent disadvantages which limit its application. For example, such a system is generally not suitable for separation of sub-micron particle sizes, since the granular filter bed material needed for sub-micron filtering must have a very fine mesh size which precludes effective backflushing because the fine bed particles themselves become entrained in the backflush gas and are blown out of the bed during backflushing. It is impractical to simply increase the thickness of the bed, since this increases the pressure drop across the bed and results in an unacceptable increase in the operating costs for the system. Another limitation of this type of system is that it is capable of removing from the system only the dust which is agglomerated into chunks large enough to be separated by the cyclone. The non-agglomerated dust tends to simply build up in the system and ultimately requires some measure to remove it.
Another commercially available granular bed filter, e.g., of the type shown in U.S. Pat. No. 4,017,278 to Reese, employs a vertically contained granular bed which is generally continuously moving. This type of system obviates the need to have separate cleaning or backflushing phases. The movement of the bed, however, is a disturbing influence which interferes with the collection of the dust in the granular bed which would otherwise occur if the entire bed were quiescent. In addition, the granules at the extreme outer edges of the filter chamber do not travel vertically downward, as does the interior of the bed, because the constant refill does not allow a drain space between the louvers of the filter walls. External dedusting of the granular material removes the separated dust from the system.
U.S. Pat. No. 4,300,921, issued Nov. 17, 1981, by the common inventor of the present invention and assigned to common assignee as the present invention provides an improved granular bed filter arrangement of the vertical type that can be employed economically to separate sub-micron dust particle sizes from a contaminated gas stream. Briefly, the device as described therein comprises a generally gas-tight containment vessel; a gas inlet for introducing the dust containing gaseous stream into the central portion of the vessel; a plurality of generally vertically oriented filter panels arranged circumferentially about the central portion of the vessel, these filter panels being adapted for receiving a bed of granular material as a filter media; a separate clean gas outlet associated with each of the filter panels, thereby defining a plurality of gas flow paths from the central portion of the vessel through each of the respective filter panels to the clean gas outlet associated with each of the filter panels. Thus the dust contained in the gas flowing through each filter panel becomes trapped in the granular filter media. Additionally, a means is provided which selectively closes each of the clean gas outlets, generally in a sequential order, to temporarily decrease the number of independent gas flow paths. Above the panels and within the vessel is a feed hopper which selectively supplies granular filter media to each of the filter panels. The panels each have a gate means which selectively empties each of the filter panels of granular filter media in response to a signal. Centrally positioned beneath the vessel is a separator which separates trapped particulate material from the granular filter media emptied from the filter panels. A conveying means communicates with the separator and conveys separated granular filter media from the separating means to the feed hopper. The separators ordinarily comprise a plurality of surfaces, preferably substantially concentrically arranged, inverted truncated conical plates, positioned below the discharge point of the means for emptying each of the filter panels of granula filter media. The surfaces are sufficiently hard that the granular filter media will bounce upon impact with the surfaces due to the force of gravity. The surfaces are inclined with respect to the vertical and are spaced from one another both vertically and horizontally in such a way that granular filter media discharged thereon will cascade downwardly from surface to surface whereas the agglomerated dust particles accompanying the granular filter media will far through the horizontal spaces between the surfaces.
It has been noted that the movement of the media due to the discharging and charging cycle in a filter device of the type described in the aforementioned commonly assigned application leaves a certain amount of airborne, very fine residual dust which becomes trapped on the clean side of a filter panel after it has been charged. When the newly charged filter panel is returned to operation, the residual dust is moved through the gas outlet to the atmosphere by the gas passing through the filter panel. Generally, the very fine dust presents no problem, but in installations where high percentage filtering is required due to environmentally sensitive concerns, the venting of residual dust to the atmosphere may require additional filtering. This, of course, is an additional expense and could well increase energy requirements for operating the filter system.
It is therefore a paramount object of the present invention to provide an improved filter system in which fine dust can be filtered without a significant concurrent increase in cost and energy requirements.
It is another object of the present invention to provide an improved vertical bed filter arrangement in which residual dust is removed prior to the return to service of the associated filter panel.
Yet another object of the invention is to provide an improved vertical bed granular media, dust filter system having means for recirculating and cleaning the media and providing a means of capturing the residual dust from the media canister.
Still another object of the present invention is to provide a means of circulating a gas stream to continually vent the housing surrounding the means for recirculating the media, for example, a bucket elevator system, back to the filter panels.
A further object of the present invention is to provide a means for enhancing the efficiency of the means for separating the dust from the discharged media by use of a gas stream.